Fix tests on kqueue() based platforms.

This patch disables fork()'ing of the slow process tests. This fixes the
tests on the MacOS and other kqueue() based platforms.

Without this patch the main loop exits eearly with EBADF as error.

See: https://bugs.torproject.org/28179

src/test/test_process_slow.c

@@ -324,7 +324,7 @@ test_callbacks_terminate(void *arg)
 }
 
 struct testcase_t slow_process_tests[] = {
-  { "callbacks", test_callbacks, TT_FORK, NULL, NULL },
-  { "callbacks_terminate", test_callbacks_terminate, TT_FORK, NULL, NULL },
+  { "callbacks", test_callbacks, 0, NULL, NULL },
+  { "callbacks_terminate", test_callbacks_terminate, 0, NULL, NULL },
   END_OF_TESTCASES
 };